List

A list is a collection of multiple objects which can be a scalar, a vector, a matrix, etc. Each object in a list can have its own name.

Create a list

To create a list, the function list() can be used as shown in the examples below.

> x <- list('a'=3, 'b'=c(1,2), 'm'=array(1:6, dim=c(3,2)))
> x
$a
[1] 3

$b
[1] 1 2

$m
     [,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6

> 
> a <- 3
> b <- c(1,2)
> m <- array(1:6, dim=c(3,2))
> 
> y <- list(a, b, m)
> y
[[1]]
[1] 3

[[2]]
[1] 1 2

[[3]]
     [,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6

> 

Access the object of a list

There are at least two ways to access the objects in a list. First, each object in the list has an index according to its order, which can be used to access that object. For example, x[[1]] will access the first object in the list. Note that [[ ]] instead of [ ] is used here. Second, the name of the object can be used to access it. For example, x$a will access a in the list. Note that a dollar sign $ is added between the name of the list and the name of the object.

> x <- list('a'=3, 'b'=c(1,2), 'm'=array(1:6, dim=c(3,2)))
> 
> x[[3]]
     [,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6
> 
> x$m
     [,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6
>